Since the beginning of the 1990s, the broadband wireless access technology has been rapidly developed with the ever-increasing demand for wireless access, in particular, broadband wireless access. The Institute of Electrical and Electronics Engineers (IEEE) formed IEEE 802.16 task group specializing in the technical standardization of broadband fixed wireless access, and its objective is to establish a universal broadband wireless access standard. In order to achieve the above objective, some world famous enterprises have further initiated Worldwide Interoperability for Microwave Access (WiMAX) Forum aiming at popularizing said standard globally.
As the next-generation wireless broadband access technology, WiMAX (IEEE 802.16) boasts accessibility that parallels wire-line high-speed data access technology. It supports various traffics by providing line-of-sight (LOS) (using a spectrum of 10-66 GHz) or non-line-of-sight (NLOS) (using a spectrum of 2-11 GHz) wireless connections between a base station and a subscriber station, including real-time traffics that have high time delay requirements, such as voice and video. As WiMAX system works at a relatively high spectrum of above 2 GHz, it usually requires a line-of-sight propagation. Thus, the coverage of the WiMAX system is seriously limited, and it is more so in urban areas where buildings are densely distributed, and the complex radio propagation not only reduces the coverage, but also produces a lot of blind areas.
The Mobile Multi-hop Relay (MMR) is a task group newly formed in March 2006 by IEEE whose major task is to define the extension of IEEE 802.16 for multi-hop relay such that the WiMAX system supports mobile multi-hop relaying. The objective of mobile multi-hop relay is throughput enhancement or coverage extension of the system without affecting the subscriber equipment. It requires that all the amendments must be compatible with IEEE 802.16e and limited to the base station and the relay station, and no change is permitted on the IEEE 802.16e user equipment. As IEEE 802.16e merely involves the physical layer (PHY) and the media access control layer (MAC), the amendments can only be carried out in the above two layers. The project authorization requirement (PAR) of MMR task group clearly requires that: (1) the relay station (RS) should be transparent to subscriber equipment; (2) the RS should be significantly smaller than the base station; and (3) the multi-hop frame structure should be only orthogonal frequency division multiple access (OFDMA)-based. Currently, preliminary studies are carried out on the definition of frame structure, network entry flow, selection and handover of relay stations, handover between the relay station and the base station, control mapping information (MAP) transportation, and the method of reporting subscriber information by the relay station, etc. Technical solutions for both throughput enhancement and coverage extension of the system are included in the ultimate standardization.
FIGS. 1A and 1B depict basic scenarios of IEEE 802.16 relay, wherein FIG. 1A schematically shows a scenario of throughput enhancement, while FIG. 1B schematically shows a scenario of coverage extension, wherein the same numerals denote the same or similar devices. For the purpose of clarification, each figure merely shows a mobile station as an example of wireless communication terminal. It is seen that in practice, a plurality of wireless communication terminals may exist simultaneously in the scenario of throughput enhancement as shown in FIG. 1A or in the scenario of coverage extension as shown in FIG. 1B. Furthermore, the wireless communication terminal is not limited to mobile station, it may also be all kinds of terminal devices which are equipped with wireless communication function, such as a personal digital assistant (PDA), a beeper, a laptop, or a portable device.
In the scenario of throughput enhancement as shown in FIG. 1A, a mobile station (MS) 103 as an example of wireless communication terminal is located within the coverage of a base station (BS) 101, and it can directly receive information broadcast by the base station 101. However, the traffic between the mobile station 103 and the base station 101 can also be forwarded through a relay station (RS) 102. It has been proved that access to the wireless communication terminal by this approach can effectively enhance the throughput of the system under certain circumstances.
In the scenario of coverage extension as shown in FIG. 1B, a mobile station 103 as an example of wireless communication device is located out of the coverage of a base station 101 and it cannot directly receive all the control information broadcast by the base station 101. Compared with conventional wireless communication terminals, a relay station 102 may have a high transmit power and a high receiving sensitivity. Hence, it is able to establish sound connection with the base station. Therefore, the mobile station 103, which is located within the coverage of the relay station 102 and out of the coverage of the base station 101, can accomplish traffic transportation with the base station 101 through the relay station 102.
Although IEEE 802.16 has depicted two application scenarios of a mobile multi-hop relay (MMR) system, so far it merely proposed the fact that the relay station should be able to relay all kinds of uplink and downlink traffics. As for the multi-hop relay system of WiMAX, the important problem to be solved lies in: how to relay traffic between the base station and the wireless device, and how to control the relay process.